The present invention relates to an electrophotographic apparatus such as an electrostatic copying machine comprising sensors for sensing copy image density and controlling the operation of the copying machine in accordance with the sensed density.
In an electrostatic copying machine it is of course required that the copy images have optimum density and contrast as well as sharpness. To attain this goal it is necessary to control various operating parameters such as the toner density (ratio of toner to carrier in a two component developer), exposure intensity, electrostatic image potential, transfer charge potential and the like.
It is known in the art to sense the toner density by measuring the permeability, electrical resistance, light reflectance, fluidity, etc. of the developer. When the toner density drops below a predetermined value due to consumption of toner in the developing process, additional toner is added to increase the toner density and thereby prevent a reduction in copy density.
However, control of toner density alone is insufficient to ensure optimum copy density, or the density of the reproduced image on the finished copy sheet. In some cases, fatigue of the carrier particles in the developer and other factors can cause the image density to decrease even if the toner density is increased. The ambient temperature and humidity as well as fatigue of the carrier particles serve to cause fluctuations in the image density.
An improved method for control of copy image density is to measure the developing ability, rather than the toner density, of the developer. This is done by measuring the optical density of a toner image of a reference mark on a non-image area of a photoconductive drum or alternatively by measuring the amount of toner which adheres to an electrically charged electrode in a developing unit. This improved method, however, is still insufficient since the copy density can change even if the developing ability of the developer is maintained constant through optimal addition of toner. For example, a reduction in the transfer charge voltage will result in a reduction in copy image density even if the developing ability of the developer is maintained constant.
Other factors which affect the copy image density independently of the developing ability of the developer are fatigue of the photoconductive drum, variations of the initial and transfer charging voltages, exposure intensity and the like. The prior art method which measures the developing ability by sensing the amount of toner accumulated on an electrode provided in a flow path of developer in the developing unit cannot maintain the copy density constant since it is independent of the exposure and transfer processes.